Suppose that all women in the world today began bearing children at replacement-level fertility rates of 2.1 children per woman. Explain why this would not immediately stop global population growth. About how long would it take for population growth to stabilize (assuming death rates did not rise)?
The effects of the birth rate per woman at a particular time are not felt until the females born then have lived out their child-bearing years. This would occur over about 20 to 40 years. The "birth rate per woman" can only be accurately established when the survey is done among women who have already completed child bearing. Women who are currently fertile must maintain the same birth rate until menopause before the full effects show up. This also takes an average of about 20 years.
To understand why global population growth would not immediately stop if all women began bearing children at replacement-level fertility rates, we must consider a few factors.
Firstly, replacement-level fertility rate refers to the rate at which, on average, each woman has enough daughters to replace herself and her partner in the population. A rate of 2.1 children per woman is generally considered necessary to achieve replacement level in countries with low infant mortality rates. However, achieving replacement-level fertility does not result in an immediate halt to population growth due to several reasons:
1. Age structure: The age structure of a population heavily influences population growth. Even if the fertility rate drops to replacement level, there will still be a considerable number of women in their childbearing years. These women might already have children or have yet to reach that stage, which means there will still be a significant increase in population due to childbirth for some time.
2. Population momentum: Population momentum refers to the tendency of population growth to continue even after fertility rates have declined. This occurs because of the existing large number of young people who will reach reproductive age in the future. As these individuals enter their childbearing years, they contribute to population growth despite reduced fertility rates.
3. Delayed impact: Even if replacement-level fertility is achieved, it takes time for the full impact of reduced fertility to be seen in population growth. This is due to the fact that each new generation of children needs to replace their parents, grandparents, and so on. Therefore, it takes a number of generations for population growth to stabilize.
Assuming death rates do not rise, it is difficult to provide an exact timeline for when population growth would stabilize, as it depends on various factors such as the initial population size, fertility rates, and population age structure. However, it is generally estimated that it would take a few decades to a century for population growth to stabilize and reach a point of equilibrium, provided that replacement-level fertility is consistently maintained.